In glassware molding, machines, such as I.S. molding machines, are employed, in which a molding assembly comprises an extruder for forming a continuous rope of molten glass; and a cutting device for cutting the molten-glass rope crosswise into a series of glass gobs.
Each glass gob is inserted inside a mold, and is pressed against the inner surfaces of the mold by a pre-forming punch device located beneath the mold and comprising a punch, and a linear piston actuator for moving the punch to and from a forward pressure position, in which the punch is inserted inside the gob.
The weight or mass of the glass gobs is controlled periodically using a gob weight control assembly, which comprises a device for detecting the position of the punch; and a control unit for adjusting the operating conditions of the extruder on the basis of a position signal from the detecting device.
In some known embodiments, the detecting device is associated with the linear piston actuator, and comprises a position transducer for determining the longitudinal position of the moving member of the piston, and therefore of the punch.
The detecting device may be designed in various ways. In some solutions, the detecting device only covers part as opposed to the whole travel of the linear actuator; and, in others, the detecting device extends completely inside the actuator, which complicates the actuator design-wise, and leaves the detecting device exposed to dirt and lubricating oils inside the actuator.
In other solutions, the detecting device itself may be so complex as to require specific, dedicated linear actuators.
The actuators normally operate in high-temperature, highly contaminated conditions, and so call for continual maintenance, which various known solutions make difficult.